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This invention relates generally to a system for sound reduction and more specifically to reducing the internal noise emanating from a gas duct by sizing the gas duct so as to maintain a laminar flow condition within the gas duct during operation of the system.
One of the difficulties with equipment and particularly with equipment confined in equipment cabinets is that the flow of air in the duct system and particularly in sheet metal duct systems produces additional noise or amplifies noise already present in the cabinet. Various known methods of reducing emitted noise include placing sound absorber materials around the outside of the ducts or attempting to cancel out sound waves through interfering sound waves.
The present invention provides an improved method and apparatus for reducing internal duct noise by maintaining a flow of a gas to and from the cabinet in a laminar flow condition while at the same time redirecting the flow away from the cabinet. While laminar flow or turbulent flow in of itself does not necessarily increase the noise level it has been found that the duct housing, which surrounds the flowing gas can flutter and chatter as turbulent gas flows though the duct. The flutter and chatter is produced by the duct sidewalls that are normally made of sheet metal bending back and forth. While duct sidewalls flexing or buckling can occur to thermal changes it has been found that a substantial amount of the duct noise is due to the result of turbulent flow conditions within the duct.
The concept of laminar and turbulent flow is known in the art. Generally, when the ratio of inertia to viscous forces is below a critical level the flow is laminar and when the ratio of inertia to viscous forces is above a critical level the flow is turbulent. The critical level is often referred to as the Reynolds number. The critical Reynolds number, where laminar flow becomes turbulent flow, can vary with conditions of the passageway. In some instance laminar flow can be maintained up to Reynolds numbers in excess of 2000 and in other cases laminar flow can be maintained only if the Reynolds number is less than 1000. In addition to the laminar flow condition and turbulent flow condition there exists an intermediate condition known as xe2x80x9cslug flowxe2x80x9d. Slug flow occurs when the flow alternates between laminar and turbulent flow. Turbulent flow and xe2x80x9cslug flowxe2x80x9d generally have pressure variations associated with the flow conditions. It should be understood that a reference to critical Reynolds number herein is meant to denote the Reynolds number where either xe2x80x9cslug flowxe2x80x9d or turbulent flow begins to occur.
It should be pointed out that while the pressure changes from laminar to turbulent flow do not necessarily directly generate unwanted noise the pressure consequences of the transformation from laminar to slug flow or to turbulent flow can cause the sidewalls of the duct to generate noise. That is, the pressures on the sidewalls of the gas duct can change abruptly as the fluid flow changes from a state of laminar flow to a state of turbulent flow or back again and the pressure can continue to fluctuate if the flow remains turbulent. As the ducts are generally lightweight material such as sheet metal it has been found that the ducts bend or buckle in response to the pressure changes thereby generating annoying duct noise.
In the present invention the gas flow within the ducts is maintained in a laminar flow condition while the gas flow is redirected from an inlet end of a gas duct to an outlet end of a gas duct. Consequently, ducts made from materials which would normally flex and flutter can now be maintained in non-buckling or a nonflexing condition to thereby minimize flow noise caused by the duct.
The invention includes a gas transfer system having a cabinet for intaking or discharging air while minimizing duct noise and in a separate embodiment a gas duct having one end for attachment to a housing with the gas duct having a smoothly curved passageway therein with each portion of the passageway having a cross sectional area which is sufficiently large so that the ratio of the inertia forces to viscous forces of the flowing gas within the passageway is sufficiently small so that a laminar flow condition is maintained throughout the gas duct.